1. Field of the Invention
The present invention relates to a video display apparatus. More particularly, this invention relates to a video display apparatus employing a color sequence control process by mixing reference colors emitted from a light source or sources included implemented by an image display apparatus in addition to suppressing the occurrence of color breakup in a color sequential display and control process, thereby matching a predefined target reference color.
2. Description of the Related Art
Even though there have been significant advances in the technologies implementing electromechanical micromirror devices such as SLMs in recent years, there are still limitations and difficulties when they are employed to provide a high quality image. Specifically, when the images are digitally controlled, the image quality is adversely affected due to the fact that the images are not displayed with a sufficient number of gray scales.
Electromechanical micromirror devices are drawing considerable interest as SLMs. Electromechanical micromirror devices consist of “a mirror array” arraying a large number of mirror elements. In general, the mirror elements, ranging from 60,000 to several million, are arrayed on a surface of a substrate in an electromechanical micromirror device. Referring to FIG. 1A for an image display system 1 including a screen 2 is disclosed in reference U.S. Pat. No. 5,214,420. A light source 10 is used for generating light energy for illuminating the screen 2. The generated light 9 is further collimated and directed toward a lens 12 by a mirror 11. Lenses 12, 13 and 14 form a beam columnator operative to columnate light 9 into a column of light 8. A spatial light modulator (SLM) 15 is controlled on the basis of data input by a computer 19 via a bus 18 and selectively redirects the portions of light from a path 7 toward an enlarger lens 5 and onto screen 2. The SLM 15 has a mirror array including switchable reflective elements 17, 27, 37, and 47 each comprising a mirror 33 connected by a hinge 30 and supported on a surface 16 of a substrate in the electromechanical mirror device as shown in FIG. 1B. When the element 17 is in one position, a portion of the light from the path 7 is redirected along a path 6 to lens 5 where it is enlarged or spread along the path 4 to impinge upon the screen 2 so as to form an illuminated pixel 3. When the element 17 is in another position, the light is redirected away from the display screen 2 and hence the pixel 3 is dark.
Most of the conventional image display devices such as the devices disclosed in U.S. Pat. No. 5,214,420 are implemented with a dual-state mirror control that controls the mirrors to operate at a state of either ON or OFF. The quality of an image display is limited due to the limited number of gray scales. Specifically, in a conventional control circuit that applies a PWM (Pulse Width Modulation), the quality of the image is limited by the LSB (least significant bit) or the narrowest pulse width as control related to the ON or OFF state. Since the mirror is controlled to operate in either an ON or OFF state, the conventional image display apparatuses have no way to provide a pulse width to control the mirror that is shorter than the control duration allowable according to the LSB. The smallest quantity of light, which determines the smallest amount of adjustable brightness for adjusting the gray scale, is the light reflected during the time duration according to the narrowest pulse width. The limited gray scale due to the LSB limitation leads to a degradation of the quality of the display image.
Specifically, FIG. 1C shows a control circuit for controlling a mirror element according to the disclosure in U.S. Pat. No. 5,285,407. The control circuit includes a memory cell 32. Various transistors are referred to as “M*” where “*” designates a transistor number and each transistor is an insulated gate field effect transistor. Transistors M5 and M7 are p-channel transistors, while transistors M6, M8, and M9 are n-channel transistors. The capacitances C1 and C2 represent the capacitive loads in the memory cell 32. The memory cell 32 includes an access switch transistor M9 and a latch 32a, which is based on a Static Random Access switch Memory (SRAM) design. The transistor M9 connected to a Row-line receives a DATA signal via a Bit-line. The memory cell 32—written data is accessed when the transistor M9 that has received the ROW signal on a Word-line is turned on. The latch 32a consists of two cross-coupled inverters, i.e., M5/M6 and M7/M8, which permit two stable states; state 1 is Node A high and Node B low and state 2 is Node A low and Node B high.
The control circuit as illustrated in FIG. 1C controls the mirrors to switch between two states and the control circuit drives the mirror to oscillate to either an ON or OFF deflected angle (or position) as shown in FIG. 1A. The minimum quantity of light controllable to reflect from each mirror element for image display, i.e., the image display gray scale resolution for a digitally controlled image display apparatus, is determined by the shortest length of time that the mirror is controllable to hold at the ON position. The length of time that each mirror is controlled to hold at an ON position is in turn controlled by multiple bit words.
FIG. 1D shows the “binary time durations” in the case of controlling SLM by four-bit words. As shown in FIG. 1D, the time durations have relative values of 1, 2, 4, and 8 that in turn determine the relative quantity of light of each of the four bits, where the “1” is the least significant bit (LSB) and the “8” is the most significant bit. According to the PWM control mechanism, the minimum quantity of light that determines the resolution of the gray scale is a brightness controlled by using the “least significant bit” for holding the mirror at an ON position during a shortest controllable length of time.
In a simple example with an n-bit word for controlling the gray scale, one frame time is divided into (2n−1) equal time slices. If one frame time is 16.7 msec. each time slice is 16.7/(2n−1) msec.
Having set these time lengths for each pixel in each frame of the image, the quantity of light in a pixel which is quantified as 0 time slices is black (no quantity of light), 1 time slice is the quantity of light represented by the LSB, and 15 time slices (in the case of n=4) is the quantity of light represented by the maximum brightness. On the basis of the quantity of light being quantified, the time of a mirror holding at the ON position during one frame duration is determined by each pixel. Thus, each pixel with a quantified value that is more than 0 time slices is displayed by the mirror holding at an ON position with the number of time slices corresponding to its quantity of light during one frame duration. The viewer's eye integrates the brightness of each pixel so that the image is displayed as if the image were generated with analog levels of light.
For controlling deflectable micromirror devices, the PWM calls for the data to be formatted into “bit-planes”, where each bit-plane corresponds to a bit weight of the quantity of light. Thus, when the brightness of each pixel is represented by an n-bit value, each frame of data has n-bit-planes. Then, each bit-plane has a 0 or 1 value for each mirror element. In the PWM described in the preceding paragraphs, each bit-plane is independently loaded and the mirror elements are controlled according to bit-plane values corresponding to them during one frame. For example, the bit-plane representing the LSB of each pixel is displayed as 1 time slice.
In the meantime, one of the color image display methods is a commonly known as a color sequential method. This is a method for dividing one frame signal into a plurality of reference color components and sequentially displaying the respective reference color component in a short period of time, thereby attaining a desired color image display. Furthermore, the reference colors use, for example, red (R), green (G) and blue (B). The color sequential method utilizes the fact that short display periods of the color components causes the reference color components to appear to be overlapping to the human eye and, thus, synthesized rather than as individual colors. This is due to limitations of the human eye.
However, if the display period of each reference color component is not sufficiently short so that the components appear synthesized to the human eye, that is, if a frame rate is not sufficiently rapid, this results in the generation of a phenomenon called color breakup (i.e., color separation) and degrades in the image quality.
Therefore, a conventional method is utilized to generate a plurality of sub-frames and then each reference color is displayed once in each sub-frame in order to increase the frame rate instead of dividing a frame signal received as input into a video display apparatus with a certain frame rate. The frame rate is simply the number of reference colors used to display the divided frames.
FIG. 2 illustrates dividing a frame 100 of a 60 Hz frame rate into six sub-frames (i.e., sub-frames 110, 120, 130, 140, 150, and 160) for a display.
Each sub-frame period is further divided into periods of red (R), green (G) and blue (B), and displayed in that order. With this configuration, a color image 105 corresponding to the frame is displayed by color images 115, 125, 135, 145, 155, and 165 for the individual sub-frames for a total of six times, which means that a color image 105 is reproduced at a 360 Hz frame rate in terms of color display.
With this color control and display process, the human eye distinguishes frames at a slower rate than the frame rate (i.e., 360 Hz in the example shown in FIG. 2) related to a color display that is the generation cycle of sub-frames, and thereby it is possible to suppress the occurrence of color breakup (i.e., color separation).
Another problem that degrades the reproducibility of a color display is when a light source included in an image display apparatus is different from the desired reference color due to an individual difference in the light source. In such a case, even if a countermeasure to degradation in image quality, such as suppressing the occurrence of color breakup (i.e., color separation) is devised (as described above), it is not possible to express, in high fidelity, a color image represented by the input frame signal. Furthermore, the above described difference is also generated by a secular change due to the light source being used for an extended period of time. Therefore, it is necessary to provide an image display apparatus allowing for the adjustment of the above described difference not only at the initial shipment of the product but also after its use has begun.